Lithography which uses an extreme ultraviolet light source for the microfabrication of next-generation semiconductors has been expected. Lithography is a technique which reduces and projects light or beams onto a silicon substrate through a mask having a circuit pattern drawn thereon and which forms an electronic circuit by exposing a resist material. The minimal processing dimensions of the circuit formed by optical lithography are basically dependent on the wavelength of the light source. Accordingly, the wavelength of the light source used for the development of next-generation semiconductors needs to be shortened, and thus a study for the development of such a light source has been conducted.
Extreme ultraviolet (EUV) is most expected as the next-generation lithography light source, and the light has a wavelength in the range of approximately 1 to 100 nm. Since the light of the range has high absorptivity with respect to all materials, and a transmissive optical system such as a lens cannot be used, a reflective optical system is used. Further, it is very difficult to develop the optical system of the EUV light range, and this optical system exhibits reflection characteristics only for a restricted wavelength.
Currently, a Mo/Si multilayer film reflection mirror with sensitivity of 13.5 nm has been developed. Then, by developing lithography techniques obtained by the combination of the light of this wavelength and the reflection mirror, it is expected that processing dimensions of 30 nm or less may be realized. In order to realize a new microfabrication technique, there is an immediate need for the development of a lithography light source with a wavelength of 13.5 nm, and radiant light from plasma with high energy density has gained attention.
The generation of light source plasma may be largely classified into laser produced plasma (LPP) using the radiation of laser and discharge produced plasma (DPP) using the discharge of a gas and driven by the pulse power technique. In DPP, the input power is directly converted into plasma energy. For this reason, the DPP has better energy converting efficiency than that of the LPP, and has an advantage in that the device is small and cheap.
The radiation spectrum from hot and highly dense plasma using the DPP is basically determined by the temperature and the density of the target material. According to the calculation result for the atomic process of the plasma, in order to obtain plasma of the EUV radiation range, the electron temperature and the electron density are respectively optimized as about several 10 eV and 1018 cm−3 in the case of Xe and Sn, and are respectively optimized as about 20 eV and 1018 cm−3 in the case of Li.
Furthermore, the plasma light source is disclosed in Non-Patent Documents 1 and 2 and Patent Document 1.